1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus on which the zoom lens can be used. More specifically, the present invention relates to a zoom lens useful as a photographic optical system of an image pickup apparatus, such as a digital still camera, a video camera, a television (TV) camera, a monitoring camera, or a silver-halide film camera.
2. Description of the Related Art
In recent years, the number of pixels per unit area in image sensors used in image pickup apparatuses, such as a digital cameras or a video cameras, has continuously increased. In order to best take advantage of these highly advanced image sensors, it is desirable that a photographic lens used in the image pickup apparatus be capable of appropriately correcting chromatic aberration as well as monochromatic (single-wavelength) aberration, such as spherical aberration or coma. In addition, it is desirable that a photographic lens used in the image pickup apparatus like this is small-sized and has a high zoom ratio. Furthermore, it is desired that a zoom lens having a high zoom ratio, whose focal length at the telephoto end is long, is capable of appropriately executing primary achromatism and also of appropriately correcting secondary spectrum.
To meet the foregoing requirements, a positive-lead type zoom lens having a high zoom ratio, which includes a lens unit having a positive refractive power at a location closest to the object side, has been used. U.S. Pat. No. 6,594,087 discusses a positive-lead type zoom lens that uses a low and anomalous dispersion material for a lens included in the first lens unit and which is capable of appropriately correcting chromatic aberration.
Japanese Patent Application Laid-Open No. 2006-113453 discusses a four-unit zoom lens including, in order from the object side to the image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a fourth lens unit having a positive refractive power. The zoom lens discussed in Japanese Patent Application Laid-Open No. 2006-113453 uses a lens made of a low dispersion material having anomalous dispersion for the third lens unit.
In addition, U.S. Pat. No. 7,304,805 discusses a five-unit zoom lens including, in order from the object side to the image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, a fourth lens unit having a negative refractive power, and a fifth lens unit having a positive refractive power. The zoom lens discussed in U.S. Pat. No. 7,304,805 uses a lens having anomalous dispersion for the first lens unit.
For a positive-lead type zoom lens, it is relatively easy to reduce the size of the entire zoom lens and to achieve a high zoom ratio at the same time. However, if the zoom ratio of a positive-lead type zoom lens is merely increased, then a large amount of secondary spectrum of axial chromatic aberration occurs in the zoom area at the telephoto end.
In order to reduce chromatic aberration, it is effective to use a lens made of a low dispersion material having anomalous dispersion at an appropriate location inside the zoom lens. In order to appropriately correct chromatic aberration, it is significant to optimally set the refractive power of each lens unit included in the zoom lens based on a characteristic of the lens material (the Abbe number, the relative partial dispersion, or the like).
In particular, in the above-described positive-lead type zoom lens including five or four lens units, unless characteristics of a material constituting a lens included in the third lens unit having a positive refractive power are appropriately set, it becomes difficult to reduce the second spectrum at the telephoto end when the zoom ratio is increased. As a result, it becomes difficult to achieve a high optical performance for the entire zooming range.